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The thing that is awful about led lamps is that most of them are run straight off the AC voltage and have massive 100% brightness flickers. If you are moving it's like a strobe. You don't see it in car lights since they are run off DC. but most, perhaps not all, AC socket lamps I've seen have really bad flicker.

I also how they have secondary lenses since LED's can be very directional the way they are typically resin cast.

Assuming the line voltage is run through a full wave bridge rectifier, there would be a 120 Hz flicker, imperceptible to most people. Toss a large capacitor across that DC output and you've got dramatically less ripple.

Your directionality comment is apropos. It's also worth noting that some people don't like the light spectrum output on white LED's. Personally, I prefer the pink tint from high pressure sodium lamps.

Assuming the line voltage is run through a full wave bridge rectifier, there would be a 120 Hz flicker, imperceptible to most people. Toss a large capacitor across that DC output and you've got dramatically less ripple.

true but then you also have 100 times the surge current when you turn them on, or a slow turn on.

What you say is of course obvious to any EE, and yet i've never actually seen a single 120v LED lamp made that way. One wonders why.

These aren't headlights, they're street lamps. Do you really care if it takes them 3 minutes to warm up?

And even assuming they have ballasts featuring accelerated warm-up, the starting current will still be as much as double the normal operating current requirements. Really though, the starting current is negligible in the grand scheme of efficiency comparisons.

I'm not an expert on line voltage LED units designed to replace incandescents, but I would imagine including a bridge rectifier and capacitor would increase the cost and pose significant design constraints due to the components size.

You don't have to have a bridge rectifier. Just run two strings of LEDs with opposing polarity, in parallel, and you have the light of one string containing the same number of LEDs, but at 120 Hz. What the bridge rectifier gains you is a fuller duty cycle, rather than one something less than 50%, and just more light from each LED. Whether you want that or not depends on heat.

What you say is of course obvious to any EE, and yet i've never actually seen a single 120v LED lamp made that way. One wonders why.

I actually have a broken LED GU10 lamp on my bench at the moment. Inside is a small bridge rectifier, a smoothing capacitor and a tiny chopper PSU. The LED string is run at around 50D. The bridge rectifier and capacitor provide around 340V (240V mains) with about 10% ripple, but the chopper compensates for this and provides a 60kHz pulse-width modulated supply which is then

Seriously, if you start your thought process with "I only pulled a B in something, but couldn't you fix..." when the people working on it have bachelor's degrees (or master's or PhD's) in the subject area, it probably would not solve the problem

I don't know what I hate more on slashdot.... seeing somebody spout off when it's obvious to anybody with even passing familiarity in the field in question that they're full of it, or seeing somebody get shat upon when they ask a perfectly valid question in an attempt to try to learn something new

I don't notice any flicker as well. I have noticed that some lights have blocks of LEDs failing rather quickly after installation though. Entire sections of turn arrows and squares in circular lights that have gone out look rather weird. It may be a brand/manufacturer issue though since I see this in the larger metro area but not in the suburb where I live even though the suburb has had them longer.

Makes you wonder if they'll have the same issues with chunks failing in these lights.

The biggest problem with LED traffic lights is that the greens are REALLY bright. You'll be shocked, especially at night how bright the damned things are. In SoCal, we have LED traffic lights everywhere.

In Québec we have them pretty much everywhere too. Sometimes when the snow is just the right consistency and falls in the right direction, it sticks to the traffic lights lenses, obscuring them. In the past, the heat generated by the incandescent bulbs would melt the snow, but the new LED lights don't produce enough heat. A city worker has to remove the snow with a kind of small broom attached to a long pole.

Older LED traffic signal conversions probably replaced the old incandescent bulbs and color filters with LED panels made of LEDs using the appropriate color. They also in some cases may have used separate AC to DC converters located either in the control box for the traffic signal which would have been on the ground near the signal. Potentially this also meant rewiring all of the lights on each pole and replacing more equipment when required. On the other hand, an AC-DC converter could have been in each

No flicker here either (in London). The oddest thing about the change as far as I was concerned was the instant on/off of the lights. You don't really think about how long it takes for incandescents to fade, however when I first saw an LED traffic light something struck me as wrong and it took me a while to work out what.

Nor here in Glasgow. What I have noticed - particularly on the amber traffic lights, because of the flashing amber phase - is that when they go off they blink out, then come back on and fade out from full brightness. I don't know why they do that - a bug in the firmware?

I've noticed that the LED tail lights fitted to all the buses now come in two types, that either go on and off instantly or on newer vehicles, they have a slight fade when they go off. Again, a Small M

LEDs for this sort of light would be surface-mount - it makes automated assembly possible. And thus they would not generally have the hemispherical plastic dome you're used to from the leaded components. They'd probably just have a transparent coating that would not bend the light much.

Well, they have been testing these lights in my home town of Scottsdale, and they have three different types installed along one stretch of road way. They are super bright, and there is no flicker whatsoever.

The fact that they are directional is an advantage in this case since they are meant to throw light in a cone shape. The ones I've seen have no secondary lens. If there is any covering at all it is completely transparent glass.

Personally I like them because the light is white, not the orange of sodium vapor. Reminds me of when I was a kid before the move from mercury vapor to sodium vapor...

Monochromatic lighting is somewhat dangerous since details don't stand out as well. A full spectrum light would be much better. However, prior to LEDs, there wasn't a good choice since you need a light that is efficient, long lasting, and durable. So LEDs not only are nice, but it really is much safer. The larger spectrum yields better detail and thus drivers able to better react to their environment.

LEDs only produce light in narrow bands of spectrum, so even if those bands are far apart, so the light looks white, the reflection from various materials may look nothing like the color seen under wide-spectrum source such as sun, incandescent or mercury vapor light.

Incorrect when talking about LEDs. "White" LEDs are covered with a phosphor that takes a blue LED's light and shifts it down. The output from the phosphor is broad spectrum, even if the original LED was a narrow band blue. Thus, these LEDs are a good wide spectrum light, instead of an approximation made from mixing red, green and blue LEDs.
Of course, the problem you described can exist, but is commonly seen only with fluorescent bulbs.

Yeah, that fucking annoys me as well. Why can't they use something like 1kHz, not 60Hz? Or can I put a strobe light that has been colored red on the back of my car?

A cars electrical system runs at 12v DC, more or less, DC being the important thing there.

They can choose any frequency they want for the tail lights, so for the parking lights, which are normally used at night, they choose something around 60-70Hz. It is like they are trying to be annoying.

I dunno. Lots of people claim they can see the 'flicker' on a CRT with a 70 hz vertical refresh rate. If I turn my head wayyyyyy to the left or right, putting the monitor in my peripheral vision, I might be able to see the flicker on a 60 hz, but never at 70 hz or higher.

Just because you don't have some trait doesn't mean that other people don't. In this case, that trait is how fast your eyes can see. Congratulations, you have slower eyes.

I am one of those people. It isn't just "flicker", I can see the image-black-image-black pattern of the CRT at 60Hz without doing any tricks like waving my hand in front of the monitor or using the side of my vision.

I can't stand to be in the same room as a CRT monitor running at 60Hz, it is almost physically painful to see. When I had a CRT I had to run it at 85Hz to be able to use it for any period of time, but still had to make the text white on black, turn the brightness down, and such.

If it doesn't bother you, then imagine replacing every CRT with a strobe light running fast, as bright as the monitor. That would be annoying and distracting, right?

Imagine that tail lights of cars and buses were red strobe lights. Around here, that is actually a reality, with most of the new buses and some new cars having tail lights running at 60Hz. It is extremely obvious to me, where I can instantly point out which cars in a long line have blinking LED tail lights.

Cars and buses with LED tail lights are not running them at 60Hz. Nor are they being pulsed at any rate. The electrical system in a car is 12V DC. The LED assembly is either designed with enough LEDs in series such that the forward voltage drop over the set is 12V (roughly 10 LEDs) or they are in parallel with a buck-style switching power supply in front of it.

Now, before you go on about how the switching power supply causes flicker, you should research how they work. You will find that for cost and siz

You will observe, instead of a smooth trail of light (referred to on film as a "motion blur") like you normally would see, you can see individual "sets" of lights; very broken partial light trails. This effect is exaggerated enough to be realized in this case by the constant movement of your eyes.

Every set of LED tail lights I've ever seen could produce this effect. How can this be? Apparently, I'm not alone in my experiences, either. I'm certainly not one of those nut jobs who says WiFi makes them nauseous, hell I can't tell teh difference between 320kbps MP3 and WAV.

I can, however, hear a high pitched whine from an old CRT with no signal, 60Hz monitor refresh gives me a headache, and LED tail lights leave a strobe pattern instead of a smooth trail.

I believe this is dangerous as it can make determining the point of origin for such types of lights difficult when split-second instinctive brain functions take over. Instead of a line leading to the current position of the tail light, in that slit second my mind has 3 "still frame" snapshots to piece the scene together with - not quite enough information.

You need to look at the LED taillights on a Cadillac in moving traffic. In the dim mode used for tail/marker lights (not the full brightness mode used when the driver presses on the brake pedal), the taillights are being dimmed by PWM with no filtering. The flicker is extremely annoying and gives a strobe-like appearance where your eyes see multiple images of the lights in moving traffic.

Why Cadillac chose to dim their LED taillights this way is beyond explanation. It makes the cars look cheap, but it ca

*sigh* You remind me of the EE graduate student who was showing me and my classmates around his lab one day. To prove the point that humans couldn't hear a 16kHz tone very well he quickly turned the power up from 1 to 11, and through pain practically paralyzed the half of the class that still had their hearing intact.

Many cars have the flickering tail LED lights, and it has nothing to do with DC-DC converters or other sources of ripple in the supply current. It's simply a matter of the duty cycle timer, the tail lights are "dimmed" not by limiting the current but by turning them completely on and off at a low frequency. The ones I've seen are in the 40-80Hz range, just stick an oscilloscope on there if you don't believe me. The flicker stops when the lights go to full illumination (i.e. the break petal is depressed).

Now go outside and look at some LED tail lights! Even if you have very poor vision you should be able to see the ones flickering at 40-50 Hz.

PS If you are legally blind, just don't comment on lighting. You are bound to suffer from foot in mouth at times; go take on those annoying audiophiles buying 1000 euro power chords.

The flicker is not in his head, it's in the taillights. I've seen the flicker, it's caused by a pulse width modulation circuit to make the taillight mode of a combination taillight/stoplight appear dimmer. A quick google search pulls up this article http://findarticles.com/p/articles/mi_m0EIN/is_2006_Nov_8/ai_n27039046 [findarticles.com] about an automotive product specifically designed to address this issue and stop the flicker by eliminating the pwm circuit. It works by reducing the DC drive to the LEDs in taillight mode instead of using pulse width modulation to reduce the average current and effective brightness.

Those tail lights were not flickering to the naked eye, it was not a police vehicle - it only showed up through the video camera. And BTW, I can see flicker too, not the extent revealed by the video, but almost imperceptibly. I know it exists. Whether it's a DC circuit or not is irrelevant, as the flickering is to do with duty cycle not frequency.

Hey, I often walked into a room and had to tell the person in front of the CRT, that his screen was X Hz. I correctly detected 60 and 75 Hz (Mostly it's 60 Hz, but I definitely can see 75 Hz). At 85 Hz it's hard, since I can't use my own CRT at 100 Hz anymore, because the graphics card is too shitty and blurs everything.

Oh, and I can hear buzzes from some PSUs, some LCDs and some chargers. I think it's the transformer, and as far as I know, transformer buzz is well know problem.

This is something I've been wondering about for awhile. LEDs (especially the white ones) are really bright for being so small, and they don't have that yellow tint that incandescent bulbs do. Compact florescent bulbs are nice, but they aren't perfect for every situation. I'm not an expert on the subject, but I've always wondered why they don't make giant LEDs that can replace ordinary light bulbs. It seems like 220 AC would be more than enough to power them.

It's being worked on. Basically the issue holding them back is cost/brightness. Given the inevitable lowering of costs of all things technological and the toxicity of CF-bulbs I think it's just a matter of a few years before LEDs take on the consumer lightbulb market in a big way.

actually, high-power LEDs such as Philips's Luxeon series are quite robust in the face of surprising amounts of heat. I've run enough current through them so that they melted their soldering several times, and while its true their efficiency declines with heat, they suffered no permanent damage. When you put an amp and half through one of those suckers, they're literally stunningly bright.

Why can't they make a single led the size of a lightbulb instead of 100 small led's.

Is it possible to make a single, huge led?

I don't know. Maybe it's the same reason that they can't make a tungsten filament the size of a whole light bulb. Instead, they keep selling us a tiny wire the size of a pubic hair surrounded by a huge void filled with argon gas. This has been going on for well over a century, and they never seem to fix it.

The white LEDS are doped to generate three distinct colors of light (R,G,B) whose combination yield a very cold blueshifted white light (>6500 K). If one seeks to use these for video, better check to see if the camera works well with such light.

Yeah. I've noticed that. What I don't get is why they choose to set the color temperature that way. Red LEDs are extremely cheap compared with producing light at the other end of the spectrum. Why in the world would they balance them towards the blue (expensive) end of the spectrum when that is both more expensive and visually unpleasant? About the only thing I can imagine about the current LED designs is that they were designed to be used in combination with standard incandescent bulbs. If you blend the two, you should get a fairly nice looking light spectrum, albeit probably a bit heavy in the yellows....

I'd buy LED lights instantly if they actually used three emitters. Unfortunately, most don't. They use two---one yellow, one blue. Because the yellow LED has a relatively narrow light spectrum compared with an incandescent, you end up with basically no light output down near the bottom of the visual spectrum. The result is light that is downright unpleasant to deal with in every way. The bluish light makes it hard to see color accurately, makes colors not reproduce well in photography or video, and really isn't good for you mood-wise. Basically, the current crop of LED lights have all the problems of CFLs except the mercury (well, and the LEDs should last a lot longer, I believe).

The question, then, becomes this: "When are we going to see properly designed white LED bulbs?"

On the other hand, while they suck for homes, the existing LED lights are perfect for street lights. First, there was one experiment [psychcentral.com] that suggests that suicides and crime may decrease when street lights are replaced with bluish lighting. Second, the color temperature of blue LEDs are virtually indistinguishable from the mercury vapor lights (~6000K) that are already used in a lot of places.

The company I work for has actually done a good amount of research on the technologies available for high efficiency lighting right now and they do indeed make warmer white LEDs. They look pretty nice and have an adequate CRI, however, their efficacy is poor enough compared to the cool white LEDs that they are in fact only about as efficient as compact florescent.

I think it has to do with the fact that the visible light generating part of white (and blue) LEDs are phosphors pumped by what is actually a ultr

Could be, but an LED that uses phosphors eliminates any interest in my book because it means the color spectrum is a spiky mess....:-) Either way, though, I'd gladly accept much less efficiency to get better light quality. I hate CFLs (even the so-called daylight CFLs) so much that I'm planning to start stockpiling incandescent bulbs soon in preparation for the U.S. ban on them. That cold, lifeless lighting just really bugs me.

Could be, but an LED that uses phosphors eliminates any interest in my book because it means the color spectrum is a spiky mess....:-)

That is the way all white LEDs work.If you want something else, you will need an RGB array of leds - those exist too, but they cost more to manufacture and can't always be used to substitute for incandescent since some applications require a single point source.

You can always put a more than one LED emitter in a single epoxy package. Tricolor LEDs are a red and green emitter inside a single clear shell. Those are at least as close to a point source as you'll ever get with a glowing filament....

First, the GP is right: most high power white LEDs are actually blue or UV LEDs with a yellow phosphor in the plastic packaging.

As for phosphors yielding a spiky mess for a spectrum: how exactly do you imagine the spectrum of an RGB LED looks? The individual primaries in such combinations are VERY narrow band, so rather than a continuous spectrum you get three distinct peaks. Phosphors are actually smoother by comparison.

White LED phosphors are blue-heavy with a yellow peak.People build them that way because they're cheap.Now that everyone's getting pissed because they look cheap, any LED module designer worth twenty cents is designing systems that have roughly 3 white to 2 red LED's to bring the spectrum down.However, every lighting designer I've talked to, when we suggest making multiple color LED fixtures, especially ones with adjustable color spectra, say "the customers *say* they want that but they won't pay for it."

LED light bulbs do exist (they're a bunch of small LEDs, not one jumbo one -- I don't know if that's feasible).

I think you hit on the problem in your post though, power. 220V (or 120V) AC certainly has enough watts, but it's not in a usable form for LEDs. They require direct current (DC) at a much lower voltage. So you need some power conversion electronics to make them work. Then, to make them work efficiently, you need more electronics to regulate the current through them. For a standard electronics project, you just use a resistor, but then you're wasting power (to the tune of P=R*I^2). Off the shelf components that regulate the power more efficiently exist, but it adds expense.

Fluorescent lights need some electronics to work too, but I don't think they're as complicated (and are thus, cheaper). Cost is a big factor here, because old incandescent light bulbs don't cost much to purchase.

The type of power supply used in LED lighting is called a 'buck/boost' converter. It is a switching supply that merely PWM's the filtered line voltage down at high frequency (40~60KHz) to the operating voltage and current of the load The difference between this and a standard switching supply is that no isolated secondary circuit is required and thus the only 'large' components are the rectified line voltage filter caps, load filter caps, choke and heatsink mounted FETs or IGBT's. This also neatly eliminate

NYC is a lost cause as far as astronomy is concerned, but I have hope that smaller cities and towns will see this and adopt it. LEDs are inheirently directional, whereas most fixtures tend to waste a lot of their light going out and up. So LEDs should be a win for astronomy.

Here in Portland, OR, we have already started to use LED street lights. And now that we're in a snow storm, these lights aren't working. LEDs don't produce heat (that's why they're efficient). By not producing heat, they don't melt the snow away from them. So all the LED streetlights in Portland are covered in snow and cannot be seen.

The old lights produce enough heat to melt all the snow. Snow in Portland is rare, so it's not that big of a deal. In NY, it's quite the opposite.

So all the LED streetlights in Portland are covered in snow and cannot be seen.

Since LEDs are more efficient (more lumens per watt) the colder their tmperature, you can at least take comfort in the fact those snow-encrusted street-lamps are very efficiently lighting up the inside of the snow.

Off-topic, but I used to do the 401 Montreal-Toronto 6-hour trip in winter fairly often. There were a few very scary times where headlights only reflected the snow back, so people turned on the 4 way flashers to get at least a periodic glimpse of where they were going.

That was back when I was young and "invincible", it scares me today to think I did that with my then wife in the car (much of the trips were basically controlled sliding requiring great reflexes).

Oh, I'm pretty sure they'll build some tiny heaters into it as well to melt the snow. Due to a mechanical glitch, you won't be able to turn them off in Summer, but hey, that way nobody needs to remember to turn them on again when the snow comes!

This is really interesting. Until I saw this I couldn't figure out the unintended downside.

LEDs are efficient (if NYC can be retrofitted for less than $2million, there are literally billions to be saved in energy across the country.)

They can be switched on and off instantly (unlike sodium or mercury vapor lights) with little reduction in life (unlike incandescents) which should allow interesting usages. Why light an empty parking lot or path until motion detectors detect someone there?

Personally I'd miss sodium vapor street lights if LED replacements became fashionable. Perhaps it is a romantic notion, but it seems to be that one of the reasons sodium lamps have become so popular is that the orange light they emit is reminiscent of fire, and in colder northern climates their warm glow is comforting to people at some deep instinctual level.

We use LED signals here in Canada(and I'm sure in places in the US) in various places. My hometown uses them and I've seen them in Toronto as well. The problem doesn't come from the signals, the problem comes from large strings of lights down the road giving back a long range flicker as you drive by. While I can't say this will be an issue for a lot of people I'm sure there's a small minority that may have an adverse reaction to them.

That being said, it may very well be less of a reaction to the current

This is just great.
In NYC right now, they're cutting the city budget, implementing a whole slew of new taxes (taxing Itunes store purchases?), cutting subway service (while hiking the rates)... and what are they spending the money on?
That's right, replacing street lights with LED bulbs. Aren't there a little more important things to worry about/spend money on right now?

The idea itself isn't that bad, considering LEDs need a lot less energy (thus money) to operate. It depends on how it's going to be implemented. If they let city workers pile up overtime to swap ALL the lights at once, it's pretty idiotic.

Phasing them out as they burn out anyway and have to be replaced, though, is a good idea.

"We don't need to belabour the advantages of LEDs over traditional lightbulbs"?

Actually, we do, since we've had lightbulbs other than incandecent for over a decade, and incandecents are never used to light streets. LEDs manage about 100 lumens per watt, similar to high pressure sodium lamps. The old orange low-pressure sodium lamps are still king of the hill at 200 lumens per watt.

So what were those advantages again? Compared to high-pressure sodium lsmps, they're the same efficiency and lifetime, but a lot more expensive. The only advantage to low pressure lamps is colour, but they loose a factor of 2 on efficiency.

The problem with LEDs at the moment is that they give off an incredibly harsh, piercing light. Not sure if they're using phosphor coating or 3-colour LEDs to achieve the white light but the slightly blueish white they produce is pretty hard on the eyes.

Another problem is how well they handle fog and rain. Current streetlamps are chosen because the wavelengths they produce penetrate fog very well. If you've a light that doesn't penetrate fog, just gets reflected, it's a complete utter nightmare as you're i

The lighting product manufacturers quote efficiency in lumens-per-watt(lpw). What they don't shove in your face in marketing is that the devil is in the details.

CFLs, LEDs, incandescents, HPS and metal halides all have drastically different spectrum outputs. Incandescents have a very broad spectrum but their lpw is astonishingly low.

CFLs have as much as 80 lpw, whereas MH and LEDs are currently at about 100 and HPS can be even higher(around 140 lpw initial, which declines over time). LEDs have the potential to be higher than HPS but across the lifetime of the HPS bulb the LED may end up with a higher average lpw and definitely much longer service life.

There are CFL's with a broader spectrum but they're less efficient. While not completely monochromatic, there is a big spectrum spike in reds and yellows for HPS bulbs. Most people find this light to be soothing. Metal halides have a broader spectrum than HPS but are less efficient than even fluorescents. There are new white LEDs in research that produce as much as 145 lpw, but these are not commercially produced yet. Philips produces a 115 lpw white LED which is available in large quantities. You're right about the blue light hazard though - phosphor based white LEDs have a large spike around 465nm.

Interested in reading more about Lighting? Read the book the pot growers read. They have the best lighting money can buy. The Best of the Growing Edge [google.com]

One thing about HPS is that it spreads light everywhere, whereas LEDs are more directed, which you want in a streetlight facing down. Omnidirectionalness can be fixed with good fixture design, but most cities use crummy fixtures.

The initial lpw on HPS is usually about 140 but this goes down as you near the end of the bulbs lifetime. LEDs have fairly consistent output until they die.

Actually, LEDs get dimmer as they get used. If they don't fail due to the semiconductor turning into molten metal, they get dimmer and dimmer and dimmer. The 100,000 hour lifetime figure on LEDs is usually the time until 50% brightness (considered to be the point where one would notice the light being dimmer).

There are many reasons for this - degradation of the junction itself, but the semiconductor itself leads to a large index of refraction - a lot of the light in a LED gets reflected back into the semiconductor. And then there's degradation of the epoxy used to seal the LED. All these conspire to make the LEDs much dimmer, and get dimmer over time.

LED traffic lights are popping up here in the UK, too. I have to agree they do not always make a good substitute for the old incandescent bulbs.

Usually with LED traffic lights unless you're looking at them dead-on they don't shine very bright, and when you get in the line of sight of some they're almost blindingly bright even from a long distance away, day or night.

Didn't anyone actually do any real world tests of these things, or at least get some opinions of regular drivers? Unbelievable!

While these things last longer, you should see the cost of these replacements, somewhere around $200 a unit. They had a trial out at the Walkley Heights depot in 2005-06 to test reliability. Sure they last longer, but the savings in power and maintenance are eaten up in the increased cost per unit.

Also you have to feel for the poor suckers who take a pole out and lumped with the bill.

Hush! Do you know how desperately I'm waiting for the city to put some of those LED lamps up so stealing lightbulbs finally gets at least halfway profitable? Today it's pointless, you don't even generate enough dough to cover for the burn ointment.